Nearly 20% of the US working population is engaged in shift work. It is well recognized that shift work is associated with an increased risk of developing cardiovascular disease and diabetes but the mechanisms underlying this elevated cardiometabolic risk remain unclear. Shift work is generally associated with sleep loss and circadian misalignment. While sleep curtailment has been recently identified as a putative novel risk factor for obesity, diabetes and hypertension, the clinical significance of the internal synchrony of endogenous rhythms is a fundamental and as yet unanswered question of circadian biology. Our group has recently completed a laboratory study showing that short-term exposure to sleep loss with circadian misalignment is associated with cardiometabolic alterations that are not reversed when sleep is aligned and extended. By comparison, alterations observed following exposure to sleep loss without circadian misalignment are fully corrected after sleep extension. These laboratory findings provided for the first time evidence in support of adverse health consequences of circadian disruption independent of sleep loss. The overall goal of the present application is to test the hypothesis that shift workers, who are chronically exposed to circadian misalignment and sleep loss, have a higher cardiometabolic risk than day workers, and that both circadian misalignment and sleep loss contribute to the elevated cardiometabolic risk. We will measure markers of cardiometabolic risk that are rapidly and profoundly affected by reduced sleep duration and/or quality and perform a detailed assessment of the phase relationships between rhythms controlled by the central circadian pacemaker and rhythms of clock gene expression in peripheral tissue. We propose to study two groups of workers (i.e. day workers, shift workers) who are employed at the University of Chicago Medical Center and who have maintained their work schedules for at least one year. Following extended ambulatory monitoring of exposure to circadian misalignment and usual sleep duration, the subjects will undergo repeated detailed laboratory assessments of 1. internal desynchrony of circadian rhythms, including rhythms of circadian gene expression in leukocytes; 2. markers of cardiometabolic risk: glucose tolerance and insulin resistance assessed by intravenous glucose tolerance testing (ivGTT), high sensitivity C-reactive protein (hsCRP), 24-h profile of heart rate variability; 3. telomere length and telomerase activity in leukocytes, which are markers of biological aging that have been linked to cardiometabolic risk factors and are influenced by lifestyle. This work is expected to demonstrate the clinical significance of circadian misalignment for the millions of individuals involved in shift work and to suggest novel strategies to improve tolerance to shift work. Public Health Relevance: Nearly 20% of the US working population is engaged in shift work, a condition associated with an increased risk of developing cardiovascular disease and diabetes. The overall goal of this project is to define the respective roles of circadian misalignment versus sleep loss in the elevated cardiometabolic risk of shift work.